https://www.youtube.com/watch?v=ADOuxqiNa1Q
As seen in this video highlighting functionality from the Polaris release for 2013, @ ~2:00, when you create a new Lead, save, and click the ‘Existing Contact’ lookup in the Qualify stage, it should automatically find a match if there is one and show the Existing Contact card and Account card in that lookup field.
I haven’t really seen this functionality in action in 2015. I just created a Contact with Name, email and Mobile - and created an exact match of that when starting a new Lead. When clicking on 'Existing Contact' it does not return the match automatically.
Is there something I am missing?